I'm still semi new and backfilling my collection, the original mk 2 became a pass and I went for the AUV, same with the mk 4 since I'll get one with the gantry.
If you're a completist back filling then grab the Mark 2 while it can be had this cheap, odds are it won't go much cheaper and it's a different suit than the AUV. The Mark 4 on the other hand is identical to the upcoming gantry release.